This invention relates generally to fuel-air optimization in annular gas turbine combustors and more particularly concerns a system for continual on-line trimming of the fuel flow rate to rings of a annular combustor to optimize NOx emissions.
FIG. 1 shows a gas turbine combustor system 10 from a Dry Low Emissions (DLE) Industrial Engine, such as the GE LM6000.TM., which includes a compressor 2, a combustor 3 and a turbine 4. Fuel is mixed with compressed air from the compressor 2 and burned in the combustor 3. The resulting flow of combustion products out of the combustor 3 drives the turbine 4, which in turn drives a load (not shown) as well as the compressor 2. The exhaust from the turbine 4 is eventually released to the atmosphere. One type of combustor commonly used today is the so-called annular combustor. One exemplary embodiment of the annular combustor comprises a plurality of separate rings, wherein each ring is connected to the compressor 2 and the fuel supply provides combustion products to drive turbine 4. This combustor is fully described in U.S. Pat. No. 5,323,604.
FIG. 1 further shows one embodiment of a DLE type annular combustor 3 having three rings 5,7, and 9. The rings define a combustion chamber (not shown) to which a fuel-air mixture from an inner ring premixer 12, a pilot ring premixer 16, and an outer ring premixer 18 is injected. Compressed air enters each of the premixers 12, 16, and 18 via an air line 13 and fuel enters via a fuel line 15. A main valve 14, also referred to as a pilot value, is disposed in the fuel line 15 to throttle the flow of fuel into each of outer ring premixer 12 and inner ring premixer 18. Alternatively, the fuel and air may be directly injected into the combustion chamber without premixing. This results in near-stoichiometric, high temperature combustion which leads to copious production of varying combinations of oxides of Nitrogen, which are generally referred to as NO.sub.x. Premixing the fuel and air prior to combustion results in lean premixed combustion, which produces lower flame temperatures and thus lower NO.sub.x emissions. Flame temperature in a ring of the combustor is proportional to the fuel-air-ratio in the operating region of a DLE type combustor, hence ring flame temperature and fuel-air-ratio are used interchangeably in the present specification.
Reducing emissions of harmful gases such as NO.sub.x into the atmosphere is of prime concern. It is, therefore, desirable for gas turbine-based power plants burning natural gas to employ means for dramatically reducing NO.sub.x emissions. Natural gas-fired gas turbines produce no measurable particulate exhaust of oxides of Sulfur (SO.sub.x) and, if the combustion process is properly controlled, very little NO.sub.x or Carbon Monoxide (CO).
There is a need for real time, on-line trimming of the fuel flow to each ring of a annular combustor in accordance with minimizing total NO.sub.x emissions. There is an additional need for a trim system to carry out the real time, on-line trimming which is retrofittable to existing gas turbines. The trim system must be such that its failure will not affect the baseline operation of the gas turbine.